In general, diagnostics in a medical or clinical setting can be aided by determining as much information about a subject as possible. For example, an increase in a patient's body temperature is often a sign of illness or infection. In another example, a pulse oximetry measurement from the patent may be used to diagnose heart defects, sleep apnea, or measure the effects of sedation.
To facilitate measuring the condition of a subject, electronic sensors may be used. In recent years, an emerging area of research has been devoted to passive sensors that can be worn by a subject. Similar to radio frequency identification (RFID) tags, these sensors are passive in the sense that they may be powered in whole or in part by an external source. In many cases, a digital circuit is also used in the sensor for data acquisition by sampling measurements taken from the subject. This approach requires the active circuitry in the passive sensor to be activated by energy harvesting from the received electromagnetic (EM) energy, then retransmit sampled data or digital ID/code to the transmitter. This classical approach requires complex circuitry in the passive sensors, thereby increasing the cost of the sensor and requiring a greater amount of wireless power to be used to power the sensor.
Previous attempts to develop an analog passive sensor have concentrated primarily on capacitive measurements (e.g., using a varactor) that alter the tuned frequency of the passive sensor. These systems can be attached over a surface acoustic wave (SAW) substrate to provide different delays for different sensors, giving a Time Domain Multiplex capability. However, the use of a capacitive transducer also limits the types of sensing techniques that may be used by the sensor. In addition, the frequency response of a capacitive analog sensor shifts with the capacitance, thereby requiring the sensor to use a larger bandwidth. This limits the number of sensors that can be interrogated within a given bandwidth.
For the reasons stated above, existing passive biosensor techniques exhibit relatively poor performance and have not seen widespread adoption. Accordingly, there is a demand for alternative approaches to developing a passive biosensor that may be worn by a subject.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.